For a chromatographic separation it is common to utilize at least three diffrent liquids, namely a sample liquid, a wash liquid, and an elution liquid.
These liquids are transferred to the chromatographic column by means of a pump, and each liquid is selected in turn by a tap or rotary valve.
The liquids will be pumped through conveniently sized tubing to the chromatographic column which may be 10 or 100 times larger in diameter than the tubing.
It is necessary that the connecting tubing is relatively small in diameter so that the liquids are delivered in a plug-flow manner. Thereby, the liquids will not experience excessive drag at the wall of the tubing or diffusion with the liquids in front and behind, respectively.
In known columns the liquids enter the distribution region of the columns in a turbulent state with a linear velocity which is normally 300-1000 times higher than the linear flow velocity through the column.
This sudden change in velocity tends to induce mixing of the liquids, and makes the sample trail onto a column during the sample loading stage.